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ANNALS NEW YORK ACADEMY OF SCIENCES 



split off as hematite, implying a certain amount of chemical reaction. 

 For the formation of such compounds as are entirely new, reactions must 

 be assumed. In this way, garnet, quartz and some of the amphibole 

 molecules must be accounted for. 



The second stage of alteration was characterized by prehnite, pectolite 

 and datolite. Of the three, prehnite and pectolite are formed of the 

 oxides to which the phase rule was applied in the earlier pages of this 

 paper ; that is, they belong to the soda-lime-alumina-silica series, to which 

 quartz, the feldspars and the zeolites belong, and their relations to pre- 

 ceding and following members of the series are of especial interest in the 

 inquiry as to the applicability of the phase rule. Datolite cannot be con- 

 sidered a member, on account of the presence of boric oxide. Its relations 

 to other minerals are of interest, however, on account of the evidence of 

 changing conditions which they offer. 



Datolite 



In the slides, the datolite is easily recognized by its moderate refraction 

 (much higher than most of the associated minerals) and brilliant polari- 

 zation tints. Prehnite is the only mineral with which it might be con- 



FiG. 16. Remnants of datolite crystals (Da) perched upon prehnite (Pr) in 

 contact with stilbite (St). X 35. Slide 101. 



fused, but this assumes a radial development or a twinning structure 

 which are unmistakable. In hand specimens, datolite is often seen in 

 large, beautifully developed crystals, not infrequently an inch or an inch 

 and a half in diameter, clear, glassy and of a light green color, showing 

 a great number of distinct crystallographic faces. The finely granular 



